The acceptable zeros of the potential of the electrical fields produced by certain dipole distributions in homogeneous volume conductors is discussed. A bridge circuit is described by which a solution of the three-arm and the four-arm central terminals of Wilson may be solved for a zero of potential of the field produced by an arbitrary distribution of dipoles in a homogeneous volume conductor. An acceptable zero of potential for evaluation of the potentials in a locus on the "body" surface distant from the heart is described.

The zero of potential of the electrical field produced by the heart beat is defined and measured as the average value over the surface of a large spherical, integrating electrode which contains the subject and a homogeneous conducting medium. The zero of potential thus defined is apparently free of assumptions of any kind and is used to evaluate the error on the three-branch and the fourbranch central terminal with equal resistances. The latter may, in certain instances, be brought to zero by using unequal resistances. The potential function at its electrodes is that of an unweighted resultant vector or a point dipole. The electrodes which contribute to its potential are remote Wherein V p is the arbitrary-dipole function for the sphere; Si is the spherical surface of a large integrating electrode (fig. 1); and V p , the average value of V p , is independent of the position of the dipole distribution within Si and is zero. Si has a radius of three feet and consists of a copper screen held in the form of a spherical surface by an exterior wood frame. An electrical hoist lowers Si into a tank containing tap water. The dipole distribution consisted of a six-pole generating electrode by which one, two, or three electromotive forces were impressed upon the volume conductor and by which the field axis was rotated in three or more noncoplanar directions. The surface Si contained several nonhomogeneous structures, From the Department of Internal Medicine, University of Oklahoma School of Medicine, Oklahoma City, Okla.Presented in part at the Twenty-Fourth Annual Postgraduate Symposium of the San Francisco Heart Association, Oct. 28, 1953. These investigations were supported by grants from the National Heart Institute of the Xational Institutes of Health, Public Health Service.Received for publication July 2S, 1953. the seat suspension, breathing tubes, face mask, contact microphones, and wire leads. Equation 1 is based on the assumption that the conducting medium within Si is homogeneous and that Ri the radius of Si, is of any value that permits Si to contain the dipole distribution. The magnitude of Ri for Si was based, however, on the idea of placing the living human subject interior to Si, figure 1. Upon immersion the potential Vo of Si must assume a value sufficiently close to the zero of the potential of the field produced by the heart beat to permit accurate evaluation of the potential at electrodes on the body surface, particularly those electrodes which are utilized with a central terminal of the Wilson type.The living human subject is a nonhomogeneous volume conductor of irregular closed surface S 2 which contains an arbitrary dipole distribution. Upon immersion a homogeneous medium (tap water) exists between the body surface S 2 and the surface »Si of the spherical integrating electrode. The field of the heart beat now extends throughout the conducting medium exterior to Si. A Helmholtz double-layer on S 2 when acting alone will produce an electrical field between Si and S 2 identical to that which is act...

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